Conventional magnetic storage technology is well advanced and a wide variety of magnetic tapes and disks have evolved. Most are erasable and have very long storage lifetimes. However, their information densities and the read/write times, while adequate for some digital systems, have been found to be limiting features when compared to the capabilities and demands of optical data processing systems using optical memory materials.
Nonerasable optical storage devices already are commercially available and have very high storage capacities and very fast read/write times. A relatively new item having found a wide range of usefulness for commercial viewing and archival record storage is the video optical disk that has high enough speeds for real-time color presentations.
Recently interest has focussed on the development of magneto-optical materials for erasable optical recording applications. A. R. Tebo, in his article in Electro-Optics 15, Number 5 (1983) 40 discusses device concepts based on the magneto-optic Kerr effect. The Kerr effect results in a rotation of the direction of polarization of a transmitted polarized beam of light depending on the state of magnetic polarization of the material. A large commercial interest has developed concerning this type of erasable optical material and several device schemes are currently under development. Most, if not all of these are based on magneto-optic materials which exhibit a reversible change in optical properties when heated with a focused laser beam. The magneto-optical materials employed rely on a magneto-optic hyseteresis effect. This means that a linearly polarized beam of light has a different direction of polarization after passing through the material, depending on whether the material at that spot has been heated above its Curie temperature in the presence of a magnetic field. While these materials have shown quite promising results with respect to storage densities and read and write times, a principle difficulty has been that, as a group, the magnetic materials tend to be physically and chemically unstable making them unable to hold information for long times.
Electro-optic effects have been observed previously in strain-biased ferroelectric ceramics, see the articles by C. E. Land, Ferroelectrics 7 (1974) 45, M. D. Drake, Applied Optics, 13 (1974) 347 and by P. J. Chen and C. E. Land Journal of Applied Physics 51 (1980) 4961. The electro-optic effects have been used in a variety of erasable memory applications employing electrically controlled scattering, birefringence, and surface deformation. Ferroelectric ceramics, however, are not suitable as digital optical memory materials which require a very thin, optically uniform films for acceptable operation.
Memories in several photoconductor-ferroelectric sandwich shells have been developed in the state-of-the-art. Representative of these are U.S. Pat. Nos. 3,902,788, 3,855,579, 3,680,060 and 3,479,651. These rely upon a photoconductive layer deposited on a ferroelectric layer and a voltage is applied across both. When unilluminated, the photoconductor is very resistive and there is only a small voltage drop across the ferroelectric layer. When illuminated at a spot the photoconductor becomes highly conductive and the ferroelectric experiences a large voltage drop and is poled at that spot. A change in the state of the ferroelectric can be detected from the rotation in the direction of polarization of a transmitted beam of light.
Optically addressable ferroelectric memory effects having refractive index modulations due to migration of photogenerated charge are represented in U.S. Pat. Nos. 3,829,845 and 3,868,653. A voltage is applied across a poled ferroelectric. The material is illuminated at a spot by light at a wavelength which photogenerates free charge in the material. The charge drifts in the direction of the electric field and eventually is retrapped changing the index of refraction of the illuminated point. The shift in index of refraction can be read as a rotation of polarization in a transmitted beam of light. The stored information can be erased by illumination at a point in a reversed electric field or by uniform illumination of the entire sample with no applied voltage.
Thus, there is a continuing need in the state-of-the-art for a ferroelectric polymer system used to optically write and erase information and read optically which relies on the recent development of thin film ferroelectric polymers having low Curie temperatures and which avoids the consequences of conventional ferroelectric ceramics. These temperatures allow a voltage applied across a thin sheet or film of the ferroelectric polymer to have a spot heated with a laser beam to a temperature such that the electric field generated by an applied voltage is sufficient to pole the material. This spot is then considered to contain a bit of information and is readable due to the effects of rotation in direction of polarization of a polarized beam of light passing through the material at that spot. The material is erasable at each spot by selective heating with a laser beam with no applied voltage to depole the material. As a consequence, very high storage densities of digital information may be made. The long term storage, erasability and rewrite capabilities are superior to magneto-optic materials.